1. Technical Field
The present invention relates to plasma reactors, for example as are used to process semiconductor wafers during wafer etching. More particularly, the present invention relates to an improved stationary focus ring for a plasma reactor.
2. Description of the Prior Art
Semiconductor wafer processing steps, such as etching or chemical vapor deposition, are often carried out using a plasma reactor in which a focus ring is placed around the wafer to reduce wafer etch non-uniformity that results from non-uniform plasma distribution across the wafer surface. It is thought that such plasma non-uniformity is caused by non-uniform process gas distribution across the wafer surface, as well as other factors, such as non-uniform cathode temperature distribution and non-uniform electrical and magnetic field distribution across the wafer.
Typically, a process gas is introduced into the chamber at the top of the chamber, and a vacuum exhaust system having an intake positioned at the bottom of the chamber draws the process gas through the chamber to maintain a desired chamber pressure. The wafer is usually supported on a pedestal placed in the center of the chamber. Process gas distribution across the wafer surface is influenced by the position of the intake of the vacuum exhaust system such that there is a higher gas flow rate at the edge of the wafer than at the wafer center. Thus, during wafer etch a higher plasma density near the wafer edges produces a significantly higher etch rate at the wafer edge than at the wafer center, thereby degrading process uniformity. A similar lack of uniformity is observed in a chemical vapor deposition ("CVD") reactor.
FIG. 1 is a partially sectioned schematic view of a prior art reaction chamber 2 having chamber walls 4 and a slit valve 6 that allows wafer ingress to and egress from the chamber. The wafer 10 is supported by a wafer pedestal 12 which is positioned over a cathode base 13. The wafer 10 is shielded by a focus ring 14 that rests on the pedestal 12 and that surrounds the wafer.
A downward process gas flow produced by a vacuum pump 8 is shown by the arrows identified by numeric designator 11. During wafer processing, the focus ring provides a partial barrier to process gas flow, such that a slight back pressure is built-up across the wafer surface, causing more process gas to flow across the wafer center. As a result a more uniform gas flow is provided across the wafer surface, thereby improving process uniformity and efficiency.
The focus ring 14 extends above the wafer surface by a few centimeters and is supported by a lift mechanism 15 that lifts the focus ring a sufficient amount to allow the wafer to be transferred between a load lock and the chamber. The lift mechanism has several moving parts, including a lift cylinder 15a that surrounds the cathode base 13, a lift spider 15b that is positioned within the cathode base, and lift pins 15c that are supported on the lift spider and that extend through vertical bores 17 that are formed in the pedestal 12.
The cathode base 13 includes voids 18 that permit vertical movement of the lift spider 15b within the cathode base. A window 15d, formed through the lift cylinder 15a permits the wafer 10 to be transported through the slit valve 6, to and from the wafer pedestal, whenever the lift cylinder is raised sufficiently to align the window with the slit valve. Whenever the lift cylinder is so raised, the lift pins 15c extend through the bores to lift the wafer above the pedestal and permit a wafer transfer blade (not shown) to slide beneath the wafer.
Although the focus ring effectively improves wafer process rate uniformity across the wafer by improving gas flow distribution across the wafer surface, the focus ring tends to trap particulate contamination 16 near the wafer edges. Such contaminants reduce die yield in that portion of the wafer near the wafer edges. Particulate contamination may also be trapped by moving parts within the chamber, for example such as comprise the lift mechanism 15. When the lift mechanism is operated, resulting vibration tends to loosen and circulate contaminating particles, thereby increasing the likelihood that such particles are deposited on the wafer surface. It is therefore desirable to reduce the complexity of the focus ring by eliminating the number of parts associated therewith, for example by producing a stationary focus ring.
It is also desirable to eliminate the build-up of particulate contamination near the wafer edges, such as is attributable to the focus ring, without surrendering any of the advantages of the focus ring. One approach to reducing particle build-up at the wafer edges is to form at least one opening in the focus ring that allows the process gas to sweep the contaminating particles from the wafer surface, through the opening and downward into the vacuum pump exhaust. Such approach is described in G. Hills, Y. J. Su, Y. Tanase, R. Ryan, Improved Focus Ring For Semiconductor Wafer Processing In A Plasma Reactor, U.S. patent application Ser. No. 08/223,335, filed Apr. 5, 1994, and commonly assigned to Applied Materials, Inc.
While the stationary focus ring described by Hills et al is meritorious, there is still a need to reduce further the effects of wafer contamination, while at the same time maintaining or improving a high degree of process uniformity across the wafer surface.